The invention relates to fixed ratio traction roller transmissions with traction rollers having lubricated hard metal contact surface areas in firm engagement with one another for the transmission of motion.
Such traction roller transmissions generally utilize large contact forces in order to prevent slippage of the rollers. Various traction or friction roller transmissions are described by Harold A. Rothbart in the "Mechanical Design and Systems" Handbook, pages 14-8 and 14-9, McGraw-Hill, New York, 1964. In the relatively simple arrangements of FIGS. 14.6 and 14.7 wherein the outer rings are slightly undersized to compress the roller arrangements, the surface pressure on the traction surfaces is always the same, that is, it is always at maximum independently of the torque transmitted through the transmission. Also, various transmission arrangements are known in which the contact pressure of the traction surfaces is dependent on the size of the torque transmitted through the transmission as, for example, in those shown on page 14-8 of said handbook.
In the arrangement according to U.S. Pat. No. 771,541, variable compressive forces are obtained by an outer ring structure so arranged that the ring is compressed at a rate depending on the torque transmitted through the transmission.
However, the arrangement requires sliding surface areas which are difficult to manufacture and unreliable in use or it requires complicated lever arrangements which amplify the reaction forces of the transmission to provide sufficient compression of the outer ring for its engagement with the traction rollers.
U.S. Pat. No. 3,610,060 shows a friction drive in which the sun roller consists of two slightly conical sections adapted to be forced toward one another when a torque is transmitted through the drive in order to provide firm engagement of the barrel-shaped planetary rollers with the sun roller and the traction ring surrounding the planetary rollers. However, the differences in curvature between the small sun roller and planetary rollers is quite large so that if the cone angle were large, substantial spin would take place which would result in wear and friction losses. On one hand, this transmission requires a relatively large cone angle of the conical sun roller faces in order to avoid their locking and, on the other hand, it requires relatively long lines of contact between the sun roller sections and the planetary rollers in order to avoid overly large surface pressures which are very localized as a result of the large curvature differences between the sun roller and planetary roller traction surfaces. As a result, such a transmission would be usable only for small power transmission requirements.